Widespread star formation throughout the Galactic center cloud Sgr B2

Transcription

1 Widespread star formation throughout the Galactic center cloud Sgr B2 and its implications for SF theory Adam Ginsburg Adam Ginsburg, 1, 2 John Bally, 3 Ashley Barnes, 4 Nate Bastian, 4 Cara Battersby, 5, 6 Henrik Beuther, 7 Crystal Brogan, 8 Yanett Contreras, 9 Joanna Corby, 8, Jeremy Darling, 3 Chris De Pree, 11 Roberto Galván-Madrid, 12 Guido Garay, 13 Jonathan Henshaw, 7 Todd Hunter, 8 J. M. Diederik Kruijssen, 14 Steven Longmore, 4 Xing Lu, 15 Fanyi Meng, 16 Elisabeth A.C. Mills, 17, 18 Juergen Ott, 19 Jaime E. Pineda, 20 Álvaro Sánchez-Monge, 16 Peter Schilke, 16 Anika Schmiedeke, 16, 20 Daniel Walker, 4, 21, 22 and David Wilner 5 Ginsburg et al 2018: tinyurl.com/sgrb2alma-2

2 There is less star formation in the CMZ than expected given the amount of dense gas Galaxies (Gao & Solomon 2004) CMZ averages (Longmore et al. 2013) Shorter τdep CMZ clouds (this study) CMZ clouds (GCMS Paper I) CMZ averages (Longmore et al. 2013) Milky Way clouds (Lada et al. 20) reduction for flatter α 3 = 2.3 IMF in CMZ representative statistical uncertainty for CMZ clouds Kauffmann Milky Way clouds (Lada et al. 20) factor reduction for atter α 3 = 2.3 IMF in CMZ representative statistic uncertainty for CMZ clouds

3 Simplified turbulent SF model Turbulence produces a lognormal distribution of gas densities High-density gas becomes gravitationally unstable and collapses Modest turbulence, representing local clouds Threshold ncrit Padoan+ 2002,2011,2014 Krumholz & McKee 2005,2012 Federrath & Klessen 2012 Hennebelle & Chabrier 2011 Hopkins 2013 Burkhart et al 2017, 2018

4 There are several star formation laws that have different predictions for the dependence of ncrit on turbulence ρ crit /ρ 0 = exp(s crit ) Laws parameterized by Federrath & Klessen 2012

5 There are several star formation laws that have different predictions for the dependence of ncrit on turbulence ρ crit /ρ 0 = exp(s crit ) Local CMZ Laws parameterized by Federrath & Klessen 2012

6 The Galactic Center Sgr B2: A starburst in our galaxy The Brick : A starless protocluster? Sgr A*: The central black hole 0 pc CMZ clouds are highly turbulent (σ=5-20 km/s)

7 [M pc 2 ] xz b [deg] plane-of-sky Kruijssen+, submitted Dale+, also submitted?

8 ALMA 3 mm continuum! (mix of free-free & dust) Ginsburg et al 2018: tinyurl.com/sgrb2alma-2

9 A visual sample of the clustered regions Ginsburg et al 2018: tinyurl.com/sgrb2alma-2

10 VLA 1.3 cm Ginsburg et al 2018: tinyurl.com/sgrb2alma-2

11 near some very faint and di use 3 mm emission; it is unclear why the 3 mm is so weak here, but it hints that there are MYSOs with 3 mm emission below our detection limit. H2 O masers Water masers are generally associated with young, accreting stars. We matched our catalog with the McGrath et al. (2004) water maser catalog, finding that 23 of our sources have a water maser within 0. These sources are likely to contain YSOs, but not necessarily MYSOs based on their H2 O maser detections alone. There are 14 masers from their catalog that do not have associated sources in our catalog, though not all of these maser spots are spatially distinct. Most of these unassociated masers are seen outside of Sgr B2 N and Sgr B2 S and may be associated with outflows. X-ray sources Some young stars exhibit X-ray emis- sion, including some MYSOs (e.g., Townsley et al. 2014), so we searched for X-ray emission from our sources. 3 of the sources have X-ray counterparts in the Muno et al. (2009) Chandra point source catalog within 0. The Muno et al. (2009) catalog covers our entire observed area. The X-ray associated sources most likely contain YSOs. There are 2 X-ray sources in the field of view that do not have associated 3 mm sources. Spitzer mid-infrared sources We searched the Yusef- Ginsburg et Zadeh et al. (2009) catalogs of 4.5 µm excess sources and YSO candidates and found only one source association, though there are 5 and 14, respectively, of these sources in our field of view. Two of the 4.5 µm excess sources and one of the YSO candidates are associated with extended H ii regions (which we do not catalog); the single association is of a 4.5 µm source with the central region of Sgr B2 M. By-eye comparison of the Spitzer maps and the ALMA images suggests that the lack of associations is at least in part because of the high extinction in al the2018: regionstinyurl.com/sgrb2alma-2 containing the 3 mm cores; there are overall nisms include free-free and thermal dust emission, so in this section we explore whether the sources could be different classes of dust or free-free sources. We examine whether they are dusty prestellar cores ( 3.3.1), externally ionized globules ( 3.3.2), H ii regions from an extended population of OB stars ( 3.3.3), or H ii regions around young massive stars ( 3.3.4). After determining that the above alternatives do not readily explain the whole sample, we conclude that the sources are primarily dense gas and dust cores with internal heating sources ( 3.3.5). YSO counting Discovered 271 point(ish) sources, most of which are new A lack of line emission We visually inspected the spec- tra extracted from the full line cubes, and no lines are detected peaking toward most of the sources (most sources have emission in some lines, such as HC3 N -9, but this emission is clearly extended and not associated with the compact source). Given the relatively poor line sensitivity (RMS 6 K), the dearth of detections is not very surprising. We therefore cannot use spectral lines to classify most sources. They are probably YSOs w/envelopes, not cores Histogram of source peak flux density

12 Class 0/I YSO: an accreting star with a gas & dust envelope may have a disk and an outflow

13 1 5 Class 0/I YSO SED Robitaille F F [relative] [relative] mm flux density 11 µm] [µm] Fig. 2. A subset of 2000 SEDs for each model set, normalized to the total luminosity of each SED. [µm] Fig. 2. A subset of 2000 SEDs for each model set, normalized to the total luminosity of each SED A11, page 9 of 16 each model set, normalized to the total luminosity of each SED. A11, page 9 of 16 A11, page 9

14 3 mm source classification Most are HMYSOs. Some are HCHIIs. All will likely form massive stars. (YSOs in Orion) L~2000 L

15 YSO counts -> density Compare to gas mass N(H2) AK 2x 23 5x x x x

16 Estimate total (proto)stellar mass using an assumed IMF each observed ~ M source implies the presence of ~0 M of lower-mass stars { Inferred Observed {

17 Local Cloud Comparison N(H2) AK 2x 23 5x x x x California Molecular Cloud (d~450 pc) AK N(H2) AK 3x x pc Lada+ 2017

18 Σstar 0.25 pc grid 0.3 pc resolution based on Gutermuth Σgas Ginsburg et al 2018: tinyurl.com/sgrb2alma-2

19 Lombardi Lada California Cloud Orion A and B based on Gutermuth Ginsburg et al 2018: tinyurl.com/sgrb2alma-2

20 Lombardi Lada California Cloud Orion A and B based on Gutermuth Ginsburg et al 2018: tinyurl.com/sgrb2alma-2

21 Lombardi Lada Lada M pc -2 3 M pc -2 Ginsburg et al 2018: tinyurl.com/sgrb2alma-2

22 Sgr B2 does not fit on Σgas-Σstar relations extrapolated from local clouds!! A linear relation! fits the Sgr B2 data, but not the local based on Gutermuth Ginsburg et al 2018: tinyurl.com/sgrb2alma-2

23 Why doesn t Sgr B2 fit on the extrapolation from local clouds? Several possibilities, most ruled out: Missing Class IIs? No, should be <4x at high Σ Overestimated Σ g? No, errors too small Multiplicity? No, requires ~5000 M /source More massive * s? No, luminosity too high Incomplete? No, would imply M*>M cloud Many LOS clouds? No, needs too many Non-uniform IMF? Only if very bottom-heavy Local cld. core->star SFE low? Maybe, but why? Higher SF threshold? Maybe. Still young. Maybe; undermines steady-state models

24 Column Density Distribution (with errors) Resolution ~ ~ 0.4 pc

25 Fraction of gas containing YSOs Pixels without stars Pixels with stars Resolution ~ ~ 0.4 pc

26 Comparison to CMZ cloud G G0.253 Sgr B2 Resolution ~ ~ 0.4 pc CDF of * s in Sgr B2

27 SF laws with ncrit falling with increasing Mach number don t match both the CMZ and local clouds (assuming CMZ clouds are more turbulent [higher M] than local) ρ crit /ρ 0 = exp(s crit ) Local CMZ Laws parameterized by Federrath & Klessen 2012

28 Summary 271 3mm sources in Sgr B2: most are HMYSOs Models of flux density distributions (luminosity functions) would be helpful Despite large YSO density, Sgr B2 does not fit on extrapolation from local clouds If there is a threshold for SF, it is higher in CMZ clouds